A laser triangulation sensor provides dimensional information about an object placed in the path of a sheet of laser light.
Referring to FIG. 1, a camera 10 placed at an oblique angle to the sheet of light 20 formed from laser 25 acquires an image 30 showing the location of the laser line on the surface of the object. In a calibrated system the location of a point x, y 40 in the camera image 30 is uniquely associated with the X,Y position 50 of the corresponding point on the surface of the object. In a typical system there are several hundred points on each laser line corresponding to the number of video lines in the camera. If the sensor system or object is translated in a controlled manner, then a complete profile of the surface of the object can be assembled from successive line scans.
The laser light sheet 20 and the camera axis form two sides of a triangle, the third being the baseline 60 between the laser and the camera—hence the term Triangulation Sensor.
The laser sheet 20 may be formed by rapidly translating a single spot with a moving mirror, or using cylindrical optics in the laser path to spread the beam in one direction to form a line. The first, dynamic system, has the advantage of high incident laser power in a single spot, but suffers form mechanical complexity, whereas the seconds static configuration, has no moving parts but can suffer from lack of incident laser power as the beam is spread over a long line and the inability to dynamically modify the intensity of laser power along the laser line.
In either configuration, the camera image of the laser on each video line comprises an intensity profile typically having a base width of 6 to 20 pixels and a height of ⅓ to full scale. The centroid of this intensity profile is the ‘x’ value of the x,y coordinate, the ‘y’ value is the number of the video line on which we are operating on. Numerous techniques exist for calculating the centroid of the laser line, including simple ½ threshold, multiple thresholds, area calculation and differentiation followed by zero crossing detection.
In all cases, some or all of the following conditions regarding the form of the pulse are assumed to be valid—                It is not clipped at peak height        It is symmetric        It has reasonable amplitude above any noise or background        It is not overly wide or narrow        
If these conditions are met, then the accuracy in determining x, the centroid, can be 1/16th pixel or better before other considerations such as speckle noise, system opto-mechanical stability, and calibration accuracy begin to be more dominant sources of error. However, if one or more of these conditions are not met, then system resolution and accuracy start to degrade in an indeterminate manner.
The two dominant dynamic conditions that affect system performance are the presence of clipping of the peak and inadequate signal amplitude above the background. The first is generally caused by too high an exposure for the surface and the other too low. The extent of this problem is several orders of magnitude as described in U.S. Pat. No. 5,811,827, issued Sep. 22, 1998, to Pryor et al., which is incorporated herein by reference as though fully set forth.
The problem can be simply stated as the system exposure should be such that the centroid peak falls within acceptable levels.
This is not a severe requirement in a dynamic laser triangulation system as the intensity of the reflected laser spot can be continually monitored and the incident laser power varied in a closed servo manner to maintain more or less constant peak heights. Fast response control circuits are required and there will always be some residual step response error if the surface exhibits abrupt reflectance or range changes.
In contrast, a static laser triangulation system has a basic limitation in that the intensity of the laser cannot be varied along the line length, only temporally for the whole line from scan to scan. It is thus impossible to provide optimum exposure conditions for many real objects at all points along the line and compromises or multiple exposures are required.
To our knowledge, current system designs do not address this issue. There are discussions about optimising system setup for best response, minimising spot size to avoid the effect of reflectance changes, providing large dynamic response detectors and post processing of Point Cloud data to remove outliers and erroneous data points, but no attempt to address the fundamental exposure problem.
Current static systems deliver centroid data when some or all of the error sources listed above are present and there are no cues provided to alert the user that the data may be suspect.
Accordingly, it is an object of the present invention to provide a laser triangulation system that addresses the disadvantages of the prior art discussed above.